Synthesis,crystallographic, and thermal properties of a new porous silica

A new porous tectosilicate has been synthesized in the presence of boric acid and 1, 2, 2, 6, 6-pentamethylpiperidine as guest molecules. It crystallizes in the monoclinic system with a_o=9.91 (1) Å, b_o=20.62 (3) Å, c_o=9.80 (2) Å, and -99.7 (2)⁰. After heat treatment at 820°C for 1 hour the guest molecules are set free whereas the silica host framework is retained. From crystal morphology and thermal behaviour it is concluded that the new material possesses channel-like voids.     

Chiral phase structure of QCD with many flavors

We investigate QCD with a large number of massless flavors with the aid of renormalization group flow equations. We determine the critical number of flavors separating the phases with and without chiral symmetry breaking in SU(N_c) gauge theory with many fermion flavors. Our analysis includes all possible fermionic interaction channels in the pointlike four-fermion limit. Constraints from gauge invariance are resolved explicitly and regulator-scheme dependencies are studied. Our findings confirm the existence of an N_f window where the system is asymptotically free in the ultraviolet, but remains massless and chirally invariant on all scales, approaching a conformal fixed point in the infrared. Our prediction for the critical number of flavors of the zero-temperature chiral phase transition in SU(3) is N_f^cr=10.0±0.29 (fermion)^+1.55_-0.63 (gluon), with the errors arising from approximations in the fermionic and gluonic sectors, respectively. PACS 11.10.Hi, 11.15.Tk, 11.30.Rd     

Studies on Clathrasils. X. The effect of “help gases” on the formation and stability of clathrasils

Members of all known clathrasil families have been synthesized from aqueous silica solutions under hydrothermal conditions in the presence of their characteristic guest molecules and in the absence of atmospheric gases. The small cages of the products, otherwise filled with nitrogen, were found to be empty. In addition to clathrasils, silica-ZSM-48 was obtained as a reaction product in the absence of atmospheric gases and pyrrolidine as the guest. It appears that small “help gas” molecules such as nitrogen play only a minor role in the stability of clathrasil frameworks unlike in the case of some clathrate hydrates. A comparison of rates of formation of clathrasils synthesized in air and in the absence of “help gases” is also presented.     

Experimentally determined oscillator strengths and linewidths for the Schumann-Runge band system of molecular oxygen—III. The (7-0) to (19-0) bands

Experimental oscillator strengths and predissociation linewidths have been derived from equivalent width measurements at room temperature for the (11-0) to (15-0) Schumann-Runge bands of molecular oxygen, and at low temperature for the (15-0) to (19-0) bands, using the Adelaide 6 m vacuum ultraviolet monochromator operated at a resolution of 0.006 nm. Photometric methods were used to measure the ultraviolet absorption for 40 groups of ratational lines, and the resulting data were interpreted using a curve of growth type of analysis to give oscillator strengths and pre-dissociation linewidths. The variation of the oscillator strengths with N? within each band was found to agree well with the theoretical predictions of Allison, while the mean band oscillator strengths derived from the results are in good agreement with previous measurements. The results demonstrate the need for accurate data about multiplet line splittings. A revised analysis of previous data for the (7-0) to (14-0) bands is also presented.     

Structure of interfacial water on fluorapatite (100) surface

The structure relaxation mechanism of the fluorapatite (100) surface under completely hydrated ambient conditions has been investigated with the grazing incidence X-ray diffraction (GIXRD) technique. Detailed information on lateral as well as perpendicular ordering corresponding to the water molecules and atomic relaxations of the (100) surface of fluorapatite (FAp) crystal was obtained from the experimental analysis of the CTR intensities. Two laterally ordered water layers are present at the water/mineral interface. The first layer consists of four water molecules located at 1.6(1) A above the relaxed fluorapatite (100) surface while the second shows the presence of only two water molecules at a distance of 3.18(10) A from the mineral surface. Thus, the first layer water molecules complete the truncated coordination sites of the topmost surface Ca atoms, while the second water layer molecules remain bonded by means of H-bonding to the first layer molecules and the surface phosphate groups. Molecular mechanics simulations using force field techniques are in good agreement with this general structural behavior determined from the experiment.     

Al-RUB-41: a shape-selective zeolite catalyst from a layered silicate

A new zeolite catalyst, Al-RUB-41, was synthesized for the first time. It was tested as a catalyst in methanol amination, and showed a shape-selective performance that results in a highly favorable product distribution. The shape-selective nature was also evidenced by using Pt-Al-RUB-41 as a bifunctional catalyst for decane hydroconversion. With its unique pore architecture and remarkable shape-selective character, Al-RUB-41 presents a significant commercial potential in industrial catalysis.     

Casimir edge effects

We compute Casimir forces in open geometries with edges, involving parallel as well as perpendicular semi-infinite plates. We focus on Casimir configurations which are governed by a unique dimensional scaling law with a universal coefficient. With the aid of worldline numerics, we determine this coefficient for various geometries for the case of scalar-field fluctuations with Dirichlet boundary conditions. Our results facilitate an estimate of the systematic error induced by the edges of finite plates, for instance, in a standard parallel-plate experiment. The Casimir edge effects for this case can be reformulated as an increase of the effective area of the configuration.     

Nucleotide pools and energy charges of smooth molluscan muscles analysed by reversed-phase,ion-pair,high-performance liquid chromatography

Summary A method is described for the analysis of the 5 mono-, di- and triphosphates of adenosine, guanosine, uridine and cytidine, as well as uridinediphosphate-glucose and cyclic AMP. Separation is achieved by reversed-phase ion-pair chromatography with linear gradient elution. Application of this method to the analysis of nucleotides in smooth molluscan muscles is described, including the determination of cAMP-levels and the calculation of energy charges for all of the four nucleotide systems.     

Protein encapsulation in mesoporous silicate: the effects of confinement on protein stability, hydration, and volumetric properties

On the basis of the predictions of statistical-thermodynamic models, it is postulated that excluded volume effects may play a significant role in the stability, interaction, and function of proteins. We studied the effects of confinement on protein un/refolding and stability. Our approach was to encapsulate a model protein, RNase A, in a mesoporous silica, MCM-48, with glasslike wall structure and with well-defined pores to create a crowded microenvironment. To the best of our knowledge, this is the first report where pressure perturbation and differential scanning calorimetric techniques are employed to evaluate the stability, hydration, and volumetric properties of the confined protein. A drastic increase in protein stability ( approximately 30 degrees C increase in unfolding temperature) is observed. The increase in stability is probably not only due to a restriction in conformational space (excluded volume effect due to nonspecific interactions) but also due to an increased strength of hydration of the protein within the narrow silica pores.     

Correlations between reported and measured ultraviolet radiation exposure of mothers and young children

Cumulative and intermittent sun exposure are risk factors for skin cancer, highlighting the need to monitor exposure during childhood. There is currently very little available information concerning the accuracy of self-reported levels of sun exposure, particularly for very young children. In this study, UV radiation (UVR) exposure measured by polysulfone dosimeters worn on the wrist was compared with a measure of estimated exposure using a diary based on recall at the end of the 4 day study period and ambient dosimetric measures. Results of the study indicate that the relative UVR exposures expressed as a fraction of daily total ambient received during the 4 day period by young children and mothers are similar. A high level of association was obtained for the estimated levels of exposure between young children and their mothers. Moderate correlations were evident for dosimeter readings of mother and child on weekends with no significant association on weekdays. The association between estimated exposure and dosimeter readings was poor and needs improvement. This may be achieved by greater consideration of structural and environmental factors that influence the levels of UVR exposure received by individuals and by increasing the level of specificity in the measurement instruments. Methodological issues such as recall of exposure after several days, quantity of sun exposure and more precise measurement of grades of exposure when outside may have more impact than previously expected, and further information is being sought with a larger sample for exposure during the summer months.